Recently, demand for digital data high-speed processing has increased. In an output circuit using MOS transistors, too, it has become required to drive a large load capacitance connected to an output terminal at a high speed. In order to increase the speed of such an output circuit for driving a large load capacitance, however, it is necessary to drive a resonant circuit formed by the load capacitance and an inductance provided by, for example, wiring conductors, with a large current driving capability. This may produce ringing in an output voltage waveform.
FIG. 1 shows a conventional CMOS transistor output circuit, using MOS transistors, which functions as an inverter. In FIG. 1, an N-channel transistor 1N developing a low (L) level output and a P-channel transistor 1P developing a high (H) level output have their respective drains connected together to an output terminal 12, and also have their gates connected together to an input terminal 11. The source of the P-channel transistor 1P is connected to a V.sub.DD voltage supply terminal 3, and the source of the N-channel transistor 1N is connected to a point of ground potential.
In the output circuit of FIG. 1, when an H-level (high level) input signal is applied to the input terminal 11, the N-channel transistor 1N is turned on and the P-channel transistor 1P is turned off so that an L-level (low level) output is developed at the output terminal 12. When an L-level input signal is applied to the input terminal 11, the P-channel transistor 1P is turned on and the N-channel transistor 1N is turned off, so that an H-level output is developed at the output terminal 12.
In order for the conventional CMOS transistor output circuit of FIG. 1 to be able to drive a load at a high speed, the transistors 1N and 1P must have large current driving capability, which usually requires that the channel of each transistor be wide. If, however, the channel width of each transistor is made larger in order to increase its current driving capability, ringing could disadvantageously occur in the output voltage waveform.
FIG. 2 is an equivalent circuit of a simplified model of an arrangement including a load to be driven coupled to the output terminal 12 of the output circuit shown in FIG. 1. The equivalent circuit of FIG. 2 corresponds to the circuit of FIG. 1 with the N-channel transistor 1N being conductive to provide the L-level output. The N-channel transistor 1N is represented by a parallel combination of a current source I1 and an ON-state resistor R.sub.ON. A load capacitance 32 (of, for example, about 100 pF) is connected through an inductance 31 (of, for example, about 20 nH) to the output terminal 12. The inductance 31 is provided by, for example, wiring by leads, copper foil wiring on a printed circuit board, or bonding wires on an integrated circuit.
As will be understood from FIG. 2, the output circuit inclusive of the load provides a resonant circuit of which the resonant frequency f.sub.o is expressed by the following equation (1). ##EQU1## where L.sub.31 is the value of the inductance 31, and C.sub.32 is the value of the capacitance 32.
Q at the resonant frequency f.sub.o is expressed by the following equation (2), where 2.pi.f.sub.o =w.sub.o. EQU Q=jw.sub.o L.sub.31 /R.sub.ON ( 2)
In this circuit, as the channel widths of the respective transistors are increased to thereby increase their current driving capability so that the output circuit can operated at a higher speed, the ON-state resistance R.sub.ON of the transistors decreases, which, in turn, makes the value of Q increase, as is understood from the equation (2). This causes the output circuit including a load to oscillate, being excited by changes of the output voltage level from H to L and from L to H, so that ringing occurs in the output waveform. FIG. 3 is an example of an output voltage waveform resulting when the output voltage level changes. This waveform has been obtained by simulation. As shown in FIG. 3, when the output voltage changes either from L to H or H to L, relatively large ringing, including a maximum voltage excursion of a magnitude of up to about 60% of the difference between the two levels, occurs. Ringing generates noise during signal transmission, which may cause an erroneous operation of a logic circuit system and may also generate undesired radiation that interferes with operations of other electronic systems.
As described above, a conventional output circuit such as the one shown in FIG. 1 has a problem that, as the current driving capability is increased to speed up the driving operation of the circuit, ringing occurs in the output voltage waveform of the circuit. In order to suppress such ringing, a damping resistor may be connected in series with the output terminal, or a certain amount of slew may be introduced into an input signal at the input terminal 11 of the output circuit (to provide slew rate control). However, such techniques are not desirable, because, in integrated circuits, in particular, a damping resistor or a slew rate control circuit requires a large area and, in case of slew rate control, a complex circuit arrangement is required.
An object of the present invention is to eliminate the above-described problem associated with conventional output circuits as described above, by providing an MOS transistor output circuit which can drive a capacitive load at a high speed and which hardly causes ringing to occur in its output voltage waveform.